1. Technical Field
The subject matter described herein relates to rechargeable electrochemical cells.
2. Discussion of Art
Some known rechargeable batteries, such as sodium metal chloride batteries, provide electric current and may be recharged based on chemical reactions occurring within one or more cells of the batteries. The cells may be referred to as electrochemical cells. The electrochemical cells may include a metal current collector or body that conveys electric current from the cell to power external electric loads and receives an electric charge to recharge the cell. A porous cathode electrode is disposed around the metal current collector. The cathode electrode may include sodium chloride. The metal current collector and the cathode electrode may be located within an ionically conducting tube. A sodium ion-conducting fluid electrolyte may be loaded into the ionically conducting tube and extends into the pores of the cathode electrode. An exterior casing encloses the cell with the cathode electrode, ionically conducting tube, and fluid electrolyte disposed within the exterior casing. An interior chamber is established between the exterior casing and the ionically conducting tube.
During discharge of electric current from the cell, the sodium metal gives up electrons and is oxidized to produce sodium ions, which pass through the ionically conducting tube and via the fluid electrolyte to the cathode electrode. The metal chloride accepts electrons and is reduced to metal and the sodium ions together with the released chloride ions from the metal chloride form sodium chloride. This reaction creates an electric potential in the cell that may be used to power external electric loads.
In order to charge the cell, a positive electric charge or current is applied to a positive terminal of the cell. The positive terminal is joined to the metal current collector such that the positive electric charge is conducted into the cathode electrode. The positive electric charge oxidizes the metal of the cathode electrode to a metal chloride and the sodium ions from sodium chloride in the interior chamber pass through the ionically conducting tube where they are reduced to sodium metal.
Known cells are hermetically sealed in ambient air at atmospheric pressure. As a result, the interior chambers include ambient air. The sodium that is collected in the interior chamber during charging of the cell can react with the oxygen in the air of the interior chamber, thereby leaving nitrogen. The remainder of the sodium generated during charge may act similar to a piston and compress the remaining nitrogen in the interior chamber. When the correct charging voltage is used to charge the cell, the amount of sodium generated from the cathode electrode is sufficiently small to avoid overly compressing the gas in the interior chamber. If an incorrect charging voltage is used, such as when a malfunctioning charge control device applies too large of a charging voltage, additional sodium may be generated within the cell. For example, additional sodium may be electrolyzed from the sodium-containing electrolyte in the cathode electrode compartment. This additional sodium can increase the pressure of the gas in the interior chamber. For example, relatively small compression of nitrogen gas in the interior chamber may result in significant increases in the pressure inside the interior chamber.
If the pressure inside the interior chamber increases too much, the ionically conducting tube can break and allow the sodium in the interior chamber to react with the cathode electrode. This reaction can cause the cell to heat up and further increase the gas pressure. The increased gas pressure can cause the outer casing of the cell to rupture.
It may be desirable to have a cell and/or energy storage device that differs from those that are currently available.